1. Field of the Invention
The present invention relates to optical glass and an optical element composed of the optical glass. More concretely, the invention relates to the optical glass suitable for press molding and the optical element composed of the optical glass.
2. Description of the Related Art
In recent years, information recording media such as DVD are widely spreading. It is eagerly desired in the market to drive pick-up lenses for reading information from the information recording media at a high speed in response to the high density of the information recording media. In order to heighten the driving speed of the pick-up lenses, it is necessary to reduce the density of optical glass to be used for the pick-up lenses. Various optical glass having small densities are conventionally proposed (For example, Japanese Patent Application Laid-Open Nos. 60-36348(1985) and 60-122747(1985)).
On the other hand, as a method of manufacturing a glass lens, a so-called press molding method is known. In this press molding method, glass which is heated to a deformation temperature (hereinafter, “At”) or higher is pressed by using a heated molding die composed of a pair of an upper die and a lower die, and lens is directly molded. This press molding method has a-smaller number of manufacturing steps than that of a conventional lens molding method of polishing glass, and thus lenses can be manufactured inexpensively for a short time. For this reason, this method is widely used as a method of manufacturing optical elements such as glass lenses in recent years.
The press molding method can be roughly divided into a reheating method and a direct press method. In the reheating method, after a gob preform or a polished preform having approximately the same shape as that of an end product is formed, the preform is again heated to a temperature not less than a softening point and is press-molded by the heated paired upper and lower dies so as to have the shape of the end product. On the other hand, in the direct press method, a molten glass drop is dropped directly onto a heated die from a glass melting furnace and is press-molded so as to have a shape of an end product. In the both press molding methods, when glass is molded, it is necessary to heat a press die to a temperature which is in the vicinity of or not less than a glass transition temperature (hereinafter, “Tg”). For this reason, as Tg of the glass is higher, the surface oxidization and a change in the metal composition of the press die occur more easily, and the life of the die becomes short, thereby causing a rise in a produce cost. When the glass is formed under an atmosphere of inactive gas such as nitrogen, deterioration of the die can be suppressed. A molding device, however, becomes complicated in order to control the atmosphere, and a running cost of the inactive gas is required, thereby increasing the produce cost. Glass whose Tg is as low as possible is, therefore, desirable as the glass to be used for the press molding methods. Further, it is desirable that the deformation temperature (At) is low similarly to Tg.
In recent years, however, people are starting to worry about the bad effects of a lead compound conventionally used to reduce Tg on the human body. For this reason, disuse of the lead compound is requested in the market. Various technologies that reduce Tg and At of glass without using the lead compound, therefore, are examined and proposed (for example, U.S. Pat. Nos. 5,744,409 and 5,919,718, Japanese Patent Application Laid-Open Nos. 2000-302479 and 2003-89543).
Although the optical glass disclosed in Japanese Patent Application Laid-Open Nos. 60-36348 (1985) and 60-122747 (1985) have a comparatively small density, their glass transition temperature is high, and thus the optical glass is not suitable for the press molding. On the contrary, the optical glass disclosed in U.S. Pat. Nos. 5,744,409 and 5,919,718, and Japanese Patent Application Laid-Open Nos. 2000-302479 and 2003-89543 has a comparatively low glass transition temperature, and thus is suitable for the press molding, but the density is large.